1. Field of the Invention
This invention relates to a digital temperature-compensated oscillator for compensating variation in the oscillation frequency caused by variation in temperature.
2. Description of the Related Art
Recently, a crystal oscillator is widely used as a reference of frequency, time and the like. In general, a crystal unit used for the crystal oscillator has a temperature coefficient and the frequency thereof varies with variation in temperature. For example, a general crystal unit of AT-cut used in a frequency range of several MHz to several ten MHz has a temperature coefficient of substantially cubic curve. The characteristic minutely varies according to the angle of cut and the inflection point thereof occurs at or near 25.degree. C.
As the precision of the electronic device becomes higher, the crystal oscillator is required to have more stable oscillation frequency. In order to meet the requirement, the crystal oscillator may be placed into a thermostatic chamber. However, when the thermostatic chamber is used, the whole size becomes larger and larger power consumption for keeping the temperature in the thermostatic chamber constant is required. Further, after the power source is turned on, it takes a long time to set the oscillation frequency stable. In addition, since parts of the crystal oscillator are exposed to relatively high temperatures of approx. 70.degree. C., high reliability may not be attained.
Further, there is provided a crystal oscillator in which a crystal unit is connected to a temperature detector and a capacitance circuit such as a thermistor paralleled with a capacitor and temperature compensation is effected according to variation in the reactance of the thermistor circuit. However, the frequency stability of this type of crystal unit is less than 1/10 of the frequency stability of the crystal unit using the thermostatic chamber.
In order to solve the above problem, a digital temperature-compensated oscillator having the construction as shown in FIG. 1 has been developed, for example. In this oscillator, a detection output of a temperature sensor 1 is supplied to a compensation voltage generating circuit 2 to generate temperature compensation voltage V.sub.co (T). The temperature compensation voltage V.sub.co (T) is added to frequency compensation voltage V.sub.f supplied to a frequency adjusting terminal 3 to derive control voltage V.sub.c (T).
The frequency compensation voltage V.sub.f is used to adjust the oscillation frequency so as to compensate for deviation of an actual oscillation frequency from the reference frequency caused by aging or the like.
The temperature compensation voltage V.sub.co (T) obtained through a resistor R.sub.1 and the frequency compensation voltage V.sub.f obtained through a resistor R.sub.2 are added together to make the control voltage V.sub.c (T).
The control voltage V.sub.c (T) is applied to a variable-capacitance diode 7 series-connected with a crystal unit 6 of a Colpitts crystal oscillator, for example, to finely adjust the oscillation frequency and keep the oscillation frequency at a constant frequency.
The control voltage V.sub.c (T) is given by the following equation. EQU V.sub.c (T)=A.times.V.sub.co (T)+B.times.V.sub.f ( 1)
where A=R.sub.2 /(R.sub.2 +R.sub.2) and B=R.sub.1 /(R.sub.1 +R.sub.2)
However, the amount of variation in the frequency for voltage applied to the variable-capacitance diode in this type of oscillator generally becomes nonlinear as shown in FIG. 2, for example. For example, a frequency offset value .DELTA.F.sub.1 caused when the control voltage V.sub.c1 is changed by an infinitesimal amount .DELTA.V.sub.c and a frequency offset value .DELTA.F.sub.2 caused when the control voltage V.sub.c2 is changed by an infinitesimal amount .DELTA.V.sub.c are different from each other. Further, the amount of variation in frequency caused when the control voltage V.sub.c is changed by a constant amount may be influenced by temperature.
For this reason, for example, even if the control voltage V.sub.c is finely adjusted at a constant temperature to cause a constant offset .DELTA.F in the oscillation frequency, the frequency offset value .DELTA.F cannot be kept constant in a wide temperature range.
FIG. 3 is a graph showing real measurements of the ratio .DELTA.F/F of the frequency offset .DELTA.F to the reference frequency F obtained when the temperature is changed from -20.degree. C. to +70.degree. C. after the offset value .DELTA.F was respectively set to 0 PPM, +2 PPM, -2 PPM, +4 PPM, and -4 PPM at a temperature of 70.degree. C. in the crystal oscillator shown in FIG. 1. As is clearly seen from the result, since the specified frequency offset .DELTA.F varies according to variation in temperature, the frequency offset cannot be kept at a constant value. In particular, the amount of variation in the offset becomes larger in a low temperature range.
Further, since the compensation characteristic by the temperature compensation voltage V.sub.co (T) is influenced when the frequency compensation voltage V.sub.f is made variable, correct temperature compensation cannot be attained.